SwingTemperatureConverter.java

// school work
import java.awt.*; //using awt's layouts
import java.awt.event.*; //using awt's event classes and listener interfaces
import javax.swing.*; //using swing's components and container

//a swing application extends from javax.swing.JFrame
public class SwingTemperatureConverter extends JFrame {
    private JTextField tfCelsius, tfFahrenheit;
    private float celsius, fahrenheit;

    public SwingTemperatureConverter() {
        //swing components should be added to the content-pane of the JFrame
        Container cp = getContentPane();
        //set this container to grid layout of 2 rows and 2 columns
        cp.setLayout(new GridLayout(2, 2, 10, 3));

        //components are added left-to-right, top-to-bottom
        cp.add(new JLabel("Celsius "));
        tfCelsius = new JTextField(10);
        tfCelsius.setHorizontalAlignment(JTextField.RIGHT);
        cp.add(tfCelsius);

        tfCelsius.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent evt) {
                celsius = Float.parseFloat(tfCelsius.getText());

                fahrenheit = (float) (9.0 / 5.0 * (celsius + 32));

                //tfFahrenheit.setText(fahrenheit + "");
                //String.format("%.1f", fahrenheit);
                tfFahrenheit.setText(String.format("%.1f", fahrenheit));
            }
        });

        cp.add(new JLabel("Fahrenheit "));
        tfFahrenheit = new JTextField(10);
        tfFahrenheit.setHorizontalAlignment(JTextField.RIGHT);
        cp.add(tfFahrenheit);

        tfFahrenheit.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                fahrenheit = Float.parseFloat(tfFahrenheit.getText());

                celsius = (float) (5.0 / 9.0 * (fahrenheit - 32));

                //tfCelsius.setText(celsius + "");
                tfCelsius.setText(String.format("%.1f", celsius));
            }
        });

        setDefaultCloseOperation(EXIT_ON_CLOSE); // for the "window-close" button
        setTitle("Swing Temperature Converter");
        setSize(300, 170);
        setVisible(true);
    }

    // The entry main() method
    public static void main(String[] args) {
        // For thread safety, use the event-dispatching thread to construct UI
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new SwingTemperatureConverter(); // Let the constructor does the job
            }
        });
    }
}